1. Field of the Invention
This invention relates to a developing method which changes an electrostatic image formed on an image retainer of an electrostatic recording apparatus such as an electrophotographic reproducing machine into a visible image, or to a developing method which changes a magnetic image into a visible image.
2. Description of the Prior Art
First of all, the reproducing process in an electrophotographic reproducing machine will be briefly described as an example.
In an electrophotographic reproducing machine of a document glass plate type, for example, a document to be reproduced is first placed on a document glass plate, and a reproduction button is then depressed so that an exposure lamp exposes and scans the document while illuminating it and keeping a predetermined relation with an optical system having reflection mirror and the like. The reflected ray of light in accordance with the density of the document is radiated onto an image retainer (photosensitive drum), which is uniformly charged electrically, through the optical system, forming an electrostatic image on the photosensitive drum. This electrostatic image is developed by a developing machine into a visible image by a toner corresponding to the density of the document.
On the other hand, a reproduction paper (transfer material) is sent from a paper feeder means in synchronism with the rotation of the photosensitive drum, is the registered to be in agreement with the toner image formed on the photosensitive member, and is thereafter transferred onto the reproduction paper by a transfer electrode. After the toner image is thus transferred, the reproduction paper is separated from the photosensitive drum and is sent to a roller fixing device. The roller fixing device consists of two rollers at least one of which is heated, and heats and fixes the toner image transferred onto the reproduction paper. Thereafter, the reproduction paper is discharged outside the main frame of the reproducing machine.
Developers used for the process described above include a two-component system developer and a one-component system developer. The two-component system consists of a toner as tinting particles and a carrier necessary for electrically charging the toner and transferring it to a developing unit, while the latter consists principally of tinting particles as a unitary structure of a resin and a magnetic substance.
A magnetic brush developing method is generally known as a developing method using a developer containing a magnetic substance.
Next, this magnetic brush developing method will be described briefly.
A developer transfer retainer or support incorporating therein a fixed or rotatable magnetic roller is disposed in the proximity of a photosensitive member. The developer is brought into sufficient contact with part of this developer transfer support. When either one, or both, of the magnetic roller and the developer transfer support rotate, the ear of the developer is formed on the peripheral surface of a sleeve, and this developer is transferred to a developing unit, where it is brought into contact with the photosensitive member. The toner particles are attracted to the charged portion of the photosensitive member, and a visible image by the toner particles is formed on the photosensitive member.
The magnetic brush developing method using the one-component system developer involves the problem that the toner particles can not be easily charged due to friction, and the aggregation of the toner particles is likely to occur. For this reason, the toner particles are not sufficiently attracted to the charged portion of the photosensitive member from time to time. This problem does not occur when the two-component system developer is used, the magnetic brush can be stably formed and the friction property of the magnetic brush with the photosensitive member is excellent. In addition, the brush exhibits a sufficient cleaning effect when used for the cleaning purpose. Accordingly, the two-component system developer has gained a wide application, although the management of the quantity of the toner particles with respect to the carrier particles is necessary. This developing method generally uses a developer consisting of magnetic carrier particles having a particle size of between several dozens to several hundreds of microns and non-magnetic toner particles having a particle size of about a dozen microns. Since the toner particles as well as the carrier particles are rather coarse, this method is not free from the problem that a high quality picture reproducing delicate lines or dots or density can not be obtained easily. In order to obtain a high quality picture by this developing methods, various attempts have been made in the past such as resin coating of the carrier particles, an improvement of a magnetic substance in the developer transfer support, the application of a bias voltage to the developer transfer support, and so forth, but these methods have not yet been entirely stable and been able to provide a sufficiently satisfactory picture. It is therefore believed that the particle sizes of the toner and carrier particles must be rather reduced, in order to obtain a high quality picture.
If the toner particles have a particle size of up to 20 .mu.m and particularly up to 10 .mu.m, the following problems occur.
(1) The influence of van der Waals forces appears to the Coulomb force at the time of development and so-called "fog" in which the toner particles are deposited to the base portion of the background of the image occurs. This fog can not be prevented easily even by the application of a D.C. bias voltage to the developer transfer support.
(2) Control of frictional charge of the toner particles becomes difficult, and the aggregation of the toner particles is likely to occur. If the particle size of the toner particles is further reduced;
(3) The carrier particles attach to the electrostatic image portion of the image retainer.
It is believed that these phenomena occur because the force of the magnetic bias drops, and the carrier particles are deposited to the image retainer together with the toner particles. When the bias voltage is increased, the carrier particles attach also to the base portion of the background of the image.
When the particle size is reduce, the undesirable side-reactions such as described above become remarkable, and a clear image can not be obtained. For this reason, it has been difficult to reduce practically the particle sizes of the toner and carrier particles.